Tor anonymity network to shrink as a result of Heartbleed flaw

The Tor Project has flagged 380 Tor relays vulnerable to the critical Heartbleed flaw to be rejected from the Tor anonymity network, reducing the network's entry and exit capacity.

The decision has already been implemented on a Tor directory authority -- a server that maintains a list of Tor relays -- controlled by Roger Dingledine, the Tor Project leader, and is likely to be followed by other directory authority operators.

The 380 relays flagged for rejection are trusted entry relays, also known as guards, and exit relays. As a result, the immediate impact of this decision would be a 12 percent reduction in the network's guard and exit capacity, Dingledine said Wednesday in an email sent to the tor-relays mailing list.

Traffic from clients typically flows through the Tor network in three hops. The first hop is through a guard relay and the final hop, before the traffic is returned on the Internet to reach its intended destination, is through an exit relay.

Twelve percent might not sound like much, but guard and exit relays play an important role on the network and are not easy to replace. Many relays are run by volunteers, but they need to be trusted and need to have enough bandwidth at their disposal to handle traffic from multiple clients.

"I thought for a while about taking away their Valid flag rather than rejecting them outright, but this way they'll get notices in their logs," Dingledine said.

It seems that the ban might be permanent. Dingledine said that he wouldn't want those relays back on the Tor network even if they upgraded their versions of OpenSSL because their operators didn't patch the flaw in a timely manner.

The Heartbleed vulnerability was announced on Apr. 7 and affects versions 1.0.1 through 1.0.1f of OpenSSL, a library that implements the TLS (Transport Layer Security) encrypted communication protocol and which is used by many operating systems, web servers, browsers and other desktop and mobile applications.

The flaw allows attackers to extract information from the memory of an application that relies on OpenSSL for TLS communications, whether that application acts as a client or a server.

Both the Tor client and relay software is potentially vulnerable if the OpenSSL library is not updated on the underlying OS.

"Tor relays and bridges could maybe be made to leak their medium-term onion keys (rotated once a week), or their long-term relay identity keys," Dingledine wrote in a blog post last week after the Heartbleed flaw was announced.

"An attacker who has your relay identity key, has your onion key, and can intercept traffic flows to your IP address can impersonate your relay (but remember that Tor's multi-hop design means that attacking just one relay in the client's path is not very useful). In any case, best practice would be to update your OpenSSL package, discard all the files in keys/ in your DataDirectory, and restart your Tor to generate new keys."

In addition to the 380 guard and exit relays that have been banned already there are over 1,000 other relays that are also vulnerable and should be added to the rejection list at some point soon, Dingledine said.

Copyright 2017 IDG Communications. ABN 14 001 592 650. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of IDG Communications is prohibited.